V8

Vyacheslav Egorov

vegorov@google.com, me@mrale.ph

guest lecture at CWP, Fall 2013

V8 is a JavaScript VM
inside Chrome and node.js

Being a JavaScript VM
is not an achievement

Being a fast one - is

What is the challenge?


// Adding integers.
function add(a, b) {
  return a + b;
}
          

// Adding doubles.
function add(a, b) {
  return a + b;
}
          

// Concatenating strings.
function add(a, b) {
  return a + b;
}
          

// Arrays are just objects with
// properties "0", "1", "2", ...
Array.prototype[1] = "ha!";
var arr = [0, /* hole */ , 2];
arr[1] // => "ha!"
          

// Making a "class".
function Dog(name, breed) {
  this.name  = name;
  this.breed = breed;
}

Dog.prototype.woof = function () {
  /* ... */
};
          

// Inheriting from a class.
function Dog(name, breed) {
  Animal.call(this, name);
  this.breed = breed;
}

Dog.prototype =
   Object.create(Animal.prototype);
          

// Another way to create a dog.
var dog = {
  name: name,
  breed: breed,
  woof: function () { /* ... */ }
};
          

// Yet another way.
function makeADog(name, breed) {
  // name and breed are now "private"
  return {
    woof: function () { /* ... */ }
  }
}
          

// Yet another way.
var Dog = {
  woof: function () { /* ... */ }
};

function makeADog() {
  var dog = Object.create(Dog);
  dog.name = name;
  dog.breed = name;
  return dog;
}
          

  • Representation
  • Resolution
  • Redundancy

Representation

Use tagging to avoid boxing everything

Pointer has last bit unused
due to alignment

pointer: xx...x1


function tag(ptr) { // ptr & 1 === 0
  return ptr | 1;
}

function untag(val) {
  return val & ~1;
}

function isPtr(val) {
  return (val & 1) === 1;
}
          

small integer: xx...x0


function tag(val) {
  return val << 1;
}

function untag(val) {
  return val >> 1;
}

function isSmi(val) {
  return (val & 1) === 0
}
          

Double arithmetic
⇒ boxing

Objects?

Hidden classes

maps from Self VM


function Point(x, y) {
  this.x = x;
  this.y = y;
}

var p1 = new Point(1, 2);
var p2 = new Point(3, 4);
          

function Point(x, y) {
  this.x = x;
  this.y = y;
}

var p1 = new Point(1, 2);
var p2 = new Point(3, 4);
          

function Point(x, y) {
  this.x = x;
  this.y = y;
}

var p1 = new Point(1, 2);
var p2 = new Point(3, 4);
          

function Point(x, y) {
  this.x = x;
  this.y = y;
}

var p1 = new Point(1, 2);
var p2 = new Point(3, 4);
          

function Point(x, y) {
  this.x = x;
  this.y = y;
}

var p1 = new Point(1, 2);
var p2 = new Point(3, 4);
          

function Point(x, y) {
  this.x = x;
  this.y = y;
}

var p1 = new Point(1, 2);
var p2 = new Point(3, 4);
          

function Point(x, y) {
  this.x = x;
  this.y = y;
}

var p1 = new Point(1, 2);
var p2 = new Point(3, 4);
          

function Point(x, y) {
  this.x = x;
  this.y = y;
}

var p1 = new Point(1, 2);
var p2 = new Point(3, 4);
          

function Point(x, y) {
  this.x = x;
  this.y = y;
}

var p1 = new Point(1, 2);
var p2 = new Point(3, 4);
          

function Point(x, y) {
  this.x = x;
  this.y = y;
}

var p1 = new Point(1, 2);
var p2 = new Point(3, 4);
p2.z = 5;
          

Approximates static structure dynamically.


var arr = [];
for (var i = 0; i < 101; i++)
  arr[i] = Math.sqrt(i);
          

Track denseness and (un)box!

Same for properties


// Want "real" fast method calls
C.prototype.methodFoo = function () {
  /* ... */
};
C.prototype.methodBar = function () {
  /* ... */
};

obj.methodFoo();
          

// Don't want (pseudo-code)
m = LoadProperty(obj, "methodFoo")
CheckIfFunction(m)
Invoke(m, obj)
          

// Want (pseudo-code)
CheckClass(obj, klass0);
Invoke(methodFoo, obj);
          

Promote functions to hidden class

Catch: The whole closure is promoted!


// What is "perf-unfriendly" here?
function buyDog() {
  return {
    woof: function () {
      /* ... */
    }
  }
}
          

different woof
different classes

objects are not always structures

  • too many properties
  • delete obj.prop

Resolution


function Load(receiver, property) {
 var O = ToObject(receiver);
 var P = ToString(property);
 var desc = O.[[GetProperty]](P);
 if (desc === $undefined) return $undefined;
 if (IsDataDescriptor(desc)) return desc.Value;
 assert(IsAccessorDescriptor(desc));
 var getter = desc.Get;
 if (getter === $undefined) return $undefined;
 return getter.[[Call]](receiver);
}

JSObject.prototype.[[GetProperty]] = function (P) {
 var prop = this.[[GetOwnProperty]](P);
 if (prop !== $undefined) return prop;
 var proto = this.[[Proto]];
 if (proto === $null) return $undefined;
 return proto.[[GetPropery]](P);
};

JSObject.prototype.[[GetOwnProperty]] = function (P) {
 return this.properties.get(P);
};
          

function Load(receiver, property) {
 var O = ToObject(receiver);
 var P = ToString(property);
 var desc = O.[[GetProperty]](P);
 if (desc === $undefined) return $undefined;
 if (IsDataDescriptor(desc)) return desc.Value;
 assert(IsAccessorDescriptor(desc));
 var getter = desc.Get;
 if (getter === $undefined) return $undefined;
 return getter.[[Call]](receiver);
}

JSObject.prototype.[[GetProperty]] = function (P) {
 var prop = this.[[GetOwnProperty]](P);
 if (prop !== $undefined) return prop;
 var proto = this.[[Proto]];
 if (proto === $null) return $undefined;
 return proto.[[GetPropery]](P);
};

JSObject.prototype.[[GetOwnProperty]] = function (P) {
 return this. properties.get(P);
};
          

function Load(receiver, property) {
 var O = ToObject(receiver);
 var P = ToString(property);
 var desc = O.[[GetProperty]](P);
 if (desc === $undefined) return $undefined;
 if (IsDataDescriptor(desc)) return desc.Value;
 assert(IsAccessorDescriptor(desc));
 var getter = desc.Get;
 if (getter === $undefined) return $undefined;
 return getter.[[Call]](receiver);
}

JSObject.prototype.[[GetProperty]] = function (P) {
 var prop = this.[[GetOwnProperty]](P);
 if (prop !== $undefined) return prop;
 var proto = this.[[Proto]];
 if (proto === $null) return $undefined;
 return proto.[[GetPropery]](P);
};

JSObject.prototype.[[GetOwnProperty]] = function (P) {
return this. properties.get(P);
};
          

tons of code
+
dictionary lookup

can we do better?

we have to do first lookup

do we have to do second?

same hidden class ⇒
same structure

Inline Caching


;; Compiled code
mov eax, obj
mov ecx, "foo"
call LoadIC_Initialize
          

// Runtime system.
function LoadIC_Initialize(obj, prop) {
  var lookupResult = obj.lookup(prop);
  patch(lookupResult.compile());
  return lookupResult.value;
}
          

// Runtime system.
function LoadIC_Initialize(obj, prop) {
  var lookupResult = obj.lookup(prop);
  patch(lookupResult.compile());
  return lookupResult.value;
}
          

;; Compiled LoadIC Stub
0xabcdef: cmp [eax - 1], klass0 jnz LoadIC_Miss mov eax, [eax + 11] ret
;; Compiled code mov eax, obj mov ecx, "foo" call 0xabcdef ;; patched!

;; Compiled LoadIC Stub
0xabcdef:
cmp [eax - 1], klass0
jnz LoadIC_Miss
mov eax, [eax + 11]
ret

;; Compiled code
mov eax, obj
mov ecx, "foo"
call 0xabcdef ;; patched!
          

;; Compiled LoadIC Stub
0xabcdef:
cmp [eax - 1], klass0
jnz LoadIC_Miss
mov eax, [eax + 11]
ret

;; Compiled code
mov eax, obj
mov ecx, "foo"
call 0xabcdef ;; patched!
          

;; Compiled LoadIC Stub
0xabcdef:
cmp [eax - 1], klass0
jnz LoadIC_Miss
mov eax, [eax + 11]
ret

;; Compiled code
mov eax, obj
mov ecx, "foo"
call 0xabcdef ;; patched!
          

;; Compiled LoadIC Stub
0xabcdef:
cmp [eax - 1], klass0
jnz LoadIC_Miss
mov eax, [eax + 11]
ret

;; Compiled code
mov eax, obj
mov ecx, "foo"
call 0xabcdef ;; patched!
          

Everything is an IC stub

  • property accesses
  • element accesses
  • method calls
  • special method calls
  • global variables lookup
  • arithmetic operations

Vector.prototype.length = function () {
  return Math.sqrt(this.x * this.x +
                   this.y * this.y);
};
          

Vector.prototype.length = function () {
  return Math.sqrt(this.x * this.x +
                   this.y * this.y);
};
          

Redundancy

Need an optimizing compiler

Crankshaft

  1. Compile unoptimized code
  2. Feed hot functions into optimizer
  3. Speculate types based on IC states
  4. Apply classic optimizations
  5. Emit optimized code

Crankshaft

  1. Compile unoptimized code
  2. Feed hot functions into optimizer
  3. Speculate types based on IC states
  4. Apply classic optimizations
  5. Emit optimized code

Checks inserted into code verify speculations

Failed check ⇒ jump to unoptimized code


Vector.prototype.length = function () {
  return Math.sqrt(this.x * this.x +
                   this.y * this.y);
};
          

CheckMap v0, klass
v1 = Load v0, @12
CheckMap v0, klass
v2 = Load v0, @12
d3 = TaggedToDouble v1
d4 = TaggedToDouble v2
d5 = Mul d3, d4
CheckMap v0, klass
v6 = Load v0, @16
CheckMap v0, klass
v7 = Load v0, @16
d8 = TaggedToDouble v6
d9 = TaggedToDouble v7
d10 = Mul d8, d9
d11 = Add d5, d10
          

CheckMap v0, klass
v1 = Load v0, @12
d3 = TaggedToDouble v1
d5 = Mul d3, d3
v6 = Load v0, @16
d8 = TaggedToDouble v6
d10 = Mul d8, d8
d11 = Add d5, d10
          
  • inlining
  • GVN
  • LICM
  • DCE
  • representation selection
  • uint32 optimization
  • escape analysis
  • type inference
  • range inference
  • bounds check elimination

There are tools to peak into optimizer

--trace-hydrogen --trace-deopt --print-opt-code --code-comments

no UI :-(